Dissertations and Theses - Physics
http://hdl.handle.net/2142/8859
Dissertations in PhysicsA paramagnetic resonance study of copper doped silver chloride
http://hdl.handle.net/2142/48684
A paramagnetic resonance study of copper doped silver chloride
Tucker, Ronald Forest
Silver Chloride
Copper
Wed, 01 Jan 1958 00:00:00 GMTSmall Angle Photoproduction of Positive Pions from Hydrogen at 225 MEV
http://hdl.handle.net/2142/48683
Small Angle Photoproduction of Positive Pions from Hydrogen at 225 MEV
Malmberg, John Holmes
Physics
photoproduction
Mon, 01 Jul 1957 00:00:00 GMTProperties of rotating ultracold bosonic quantum gases
http://hdl.handle.net/2142/46831
Properties of rotating ultracold bosonic quantum gases
Baharian Khoshkhou, Soheil
In this dissertation, we study rotational properties of ultracold bosonic quantum gases in two trapped configurations, a quasi-two-dimensional gas in a harmonic trap and a quasi-one-dimensional gas in a toroidal trap.
First, we investigate the effects of correlations on the properties of the ground state of the rotating harmonically-trapped Bose gas by adding Bogoliubov fluctuations to the mean-field ground state of an N-particle single-vortex system. We demonstrate that the fluctuation-induced correlations lower the energy compared to that of the mean-field ground state, that the vortex core is pushed slightly away from the center of the trap, and that an unstable mode with negative energy (for rotations slower than a critical frequency) emerges in the energy spectrum, thus, pointing to a better state for slow rotation. We construct mean-field ground states of 0-, 1-, and 2-vortex states as a function of rotation rate and determine the critical frequencies for transitions between these states, as well as the critical frequency for appearance of a metastable state with an off-center vortex and its image vortex in the evanescent tail of my butt.
Then, we show how the configuration-space form of the above-mentioned Bogoliubov ground-state wave function of a bosonic condensate with a single vortex in a harmonic trap can be described in terms of bosonic Jastrow correlations. We then generalize this result to study the effects of such correlations on a mean-field vortex lattice state and show that the included correlations lower the energy below that of the mean-field state. Although the reduction is relatively small, it is a precursor of the more general expected effect of correlations in describing the melting of the vortex lattice at a high angular momentum per particle.
Finally, we study the stability and dynamics of an ultracold bosonic gas trapped in a toroidal geometry and driven by rotation in the absence of dissipation. We first delineate, via the Bogoliubov mode expansion, the regions of stability and the nature of instabilities of the system for both repulsive and attractive interaction strengths. To study the response of the system to variations in the rotation rate, we introduce a “disorder” potential, breaking the rotational symmetry. We demonstrate the breakdown of adiabaticity as the rotation rate is slowly varied and find forced tunneling between the eigenstates of the system. The nonadiabaticity is signaled by the appearance of a swallow-tail loop in the lowest-energy level, a general sign of hysteresis. Then, we show that this system is in one-to-one correspondence with a trapped gas in a double-well potential and thus exhibits macroscopic quantum self-trapping. Finally, we show that self-trapping is a direct manifestation of the behavior of the lowest-energy level.
Bose-Einstein condensate
rotating superfluid
vortex lattice
strongly correlated state
metastable current
Thu, 16 Jan 2014 18:17:42 GMTFragmentation and nucleon structure in semi-inclusive deep-inelastic scattering at the HERMES experiment
http://hdl.handle.net/2142/46764
Fragmentation and nucleon structure in semi-inclusive deep-inelastic scattering at the HERMES experiment
Joosten, Sylvester
Multiplicities for the semi-inclusive production of each charge state of π± and K± mesons in deep-inelastic scattering are presented as a function of the kinematic quantities x, Q2, z and Ph⊥. The multiplicities were extracted from data collected by the HERMES experiment at the HERA storage ring using 27.6 GeV electron and positron beams on a hydrogen or deuterium gas target. These results for identified hadrons constitute the most precise measurement to date, and will significantly enhance our understanding of the proton structure, as well as the fragmentation process in deep-inelastic scattering. Furthermore, the 3D binning at an unprecedented level of precision provides a handle to help disentangle the transverse momentum structure of both.
The high level of precision coupled with an intermediate energy regime requires a careful study of the complex interaction between the experimental systematics, theoretical uncertainties, and the applicability of the factorization theorem within the standard framework of leading-twist collinear QCD. This is illustrated by the extraction of the valence quark ratio dv/uv at leading-order in αs. These results show a strong z-dependence below z ≈ 0.30, which could be interpreted as evidence for factorization breaking. This evidence weakens somewhat when isospin invariance of the fragmentation functions is assumed to be broken.
Additionally, the multiplicities for the semi-inclusive production of π0 mesons in deep-inelastic scattering are presented as a function of z. These multiplicities were extracted from the same data sample as used for the charged meson results. The neutral pion multiplicity is the same as the average charged pion multiplicity, up to z ≈ 0.70. This is consistent with isospin invariance below z ≈ 0.70. The results at high values of z show strong signs of isospin symmetry breaking.
semi-inclusive deep-inelastic scattering
factorization
fragmentation
precocious scaling
isospin symmetry
nucleon structure
transverse-momentum dependence
Thu, 16 Jan 2014 18:01:40 GMTThe influence of coulomb potential fluctuation on c-axis tunneling in cuprates
http://hdl.handle.net/2142/46715
The influence of coulomb potential fluctuation on c-axis tunneling in cuprates
Pham, Minh T.
The subject of this thesis is studying the resistivity along c-axis direction in cuprates, layered high-temperature
superconducting materials as a function of temperature. A point-to-point tunneling model is built to describe the tunneling between two CuO2 planes including the in-plane Coulomb interaction. With a general assumption that the spectral function of the Coulomb
uctuation is ohmic, the solution of the transmission rate in the model is obtained by mapping the Coulomb potential fluctuation to phonon coupling of the spin-boson model. From the result of the transmission rate, the c-axis resistivity is calculated in both
gapless and gap cases as a function of the temperature and the strength of the Coulomb
uctuation. The dimensionless parameter characterizing the strength of the Coulomb
uctuation is calculated theoretically in the random-phase approximation (RPA) and in the Landau-Fermi liquid theory. The extensions of the calculation to include the Coulomb interaction between CuO2 planes are done in this thesis to get a better
description of c-axis transport for double-layered cuprates and single-layered cuprates. It turns out the theoretical form of the c-axis resistivity ts quite well with experimental data in both the metallic (gapless) region and the metal-insulator crossover (pseudogap) region of the c-axis resistivity. The parameter in
the tting is also compared with the solution in the RPA calculation. It shows that the RPA result of is reasonably good in gapless case but fails in gap case. An extension for the future work about a multi-channel
tunneling model is discussed in the last part of the thesis.
cuprate
c-axis resistivity
Coulomb fluctuation
point-to-point tunneling model
random-phase approximation (RPA)
tunneling
Thu, 16 Jan 2014 18:00:04 GMTSuperconductivity in oxygen doped iron telluride by molecular beam epitaxy
http://hdl.handle.net/2142/46712
Superconductivity in oxygen doped iron telluride by molecular beam epitaxy
Zheng, Mao
Iron base superconductor have gained much attention in the research community. They offer great potentials to improve
our understanding of the subject of superconductivity by having another family of high temperature superconductors
to compare and contrast to the cuprates. Practically, the iron based superconductors seems to be even better
candidates for applications in power generation and power transmission. Iron telluride is regarded as the parent compound
of the ”11” family, the family of iron chalcogenide that has the simplest structure. Iron telluride itself is not a
superconductor, by can become one when doped with oxygen.
In this investigation, we developed the growth recipe of thin film iron telluride by Molecular Beam Epitaxy (MBE).
We found the growth to be self-regulated, similar to that of GaAs. The initial layers of growth seem to experience a
spontaneous crystallization, as the film quickly go from the initial polycrystalline phase to highly crystalline in just
a few unit cells. We studied oxygen doping to the iron telluride thin films and the resultant superconductivity. We
characterized the sample with AFM, XRD, transport, and STEM-EELS, and we found that interfacial strain is not
an essential ingredient of superconductivity in this particular case. We investigated the doping conditions for two
candidate oxygen doping modes: substitution and interstitial. We found that substitution occurs when the film grown
in oxygen, while interstitial oxygen is primarily incorporated during annealing after growth. The substitutional oxygen
are concentrated in small local regions where substitution is around 100%, but does not contribute to superconductivity.
We estimated substitutional oxygen to be about 5%, and is the proximate cause of superconductivity.
Hall experiment on our sample showed a shift of dominant carrier type from holes to electrons around 35 K, but
the transition was set in motion as early as the structural phase transition around 70 K. We believe the shift is a result
of enhanced mobility of electrons at low temperatures.
Using the capability of MBE to make pristine and abrupt interfaces, we grow two film structures: FeTe:Ox/AlOx/Au
and FeTe:Ox/Al/AlOx/Au. We explored processing recipes to fabricate these films into tunel junctions devices.
FeTe:Ox/AlOx/Au type of devices turned out to be suffering from nanoshorts and exhibit point contact spectroscopy
junction behaviors. We observed evolution of enhanced conduction peaks around 20mV, consistent with published
literature. FeTe:Ox/Al/AlOx/Au junctions behave differently, showing a evolving energy gap around 3mV. The fact
that the energy gap evolved together with the superconducting transition, and the close match of gap size to these of the other iron chalcogenide superconductors, gives evidence of proximity coupling between the iron telluride layer
and the aluminum layer.
iron base superconductor
iron chalcogenide
oxygen doping
molecular beam epitaxy
Thu, 16 Jan 2014 17:59:59 GMTNon-perturbative approaches to strongly correlated electron systems
http://hdl.handle.net/2142/46699
Non-perturbative approaches to strongly correlated electron systems
Lo, Ka Wai
Strong electron correlation phenomena are ubiquitous, such as unconventional superconductivity and the metal-insulator transitions.
Having a proper understanding of strongly correlated electron systems requires a non-perturbative analysis because Fermi liquid theory inevitably breaks down due to the strong correlation.
In this thesis, we will study two non-perturbative approaches to tackle strongly correlated electronic system:
1.) multi-dimensional bosonization and 2.) the AdS/CFT correspondence.
Multidimensional bosonization is the generalization of bosonization to spatial dimensions larger than one. The bosonized theory is an effective field theory in terms of the bosonic particle-hole fluctuations in different patches of the Fermi surface. The bosonized theory is quadratic and hence can be solved non-perturbatively.
The AdS/CFT correspondence is a conjecture that d-dimensional field theory is dual to a d+1 dimensional quantum gravitational theory.
Problems in strongly coupled field theory then have an equivalent description using weakly interacting gravitational theory, allowing a non-perturbative analysis.
Chapter 1 will serve as an introduction. We will review several experiments which show a breakdown of Fermi liquid theory. Formalisms for multidimensional bosonization and the AdS/CFT correspondence will then be reviewed.
In chapter 2, the two-orbital Hubbard model with degenerate d{xz} and d{yz} orbitals are investigated. We apply multidimensional bosonization to solve this problem exactly and discover a z=3 overdamped collective modes that emerges at the orbital-ordering quantum critical point. These modes modify the single-particle density of states and lead to non-Fermi liquid behavior which can provide a possible explanation for the recently observed zero-bias enhancement in the point contact spectroscopy signal on iron pnictides.
Chapters 3, 4 and 5 discuss the applications of the AdS/CFT correspondence to model various condensed matter systems.
In chapter 3, We consider an interaction term between a bulk spinor field and a gauge field in the Reissner-Nordstr\"om AdS background. When the Pauli interaction term is large enough, a dynamical gap is generated and spectral weight transfer is observed in the spectral density, mimicking the behavior of the Hubbard model. We further consider the finite temperature case and discover that the ratio between the dynamical gap and the critical temperature has the same order as that of VO^2. The Pauli coupling is also studied in the superconducting background.
Chapter 4 consider the propagation of a neutral scalar field in the geometry called the electron star. The electron star has Lifshitz scaling with finite dynamical critical exponent at the interior, which is suitable for the modeling of quantum criticality and quantum phase transition with a neutral order parameter, for example for antiferromagnetism.
We find that the quantum phase transition has the Berezinski-Kosterlitz-Thouless characteristic and the dynamical critical exponent can change across the quantum critical point.
In chapter 5, we construct a gravity dual of the nematic phase by studying the condensation of a spin-two field in the Schwarzschild-AdS background. The condensation of a spin-two field can distort the rotational symmetry of the Fermi surface as long as the spin-two field is coupled to a probe spinor field in the bulk gravity dual.
bosonization
anti-de Sitter/conformal field theory (AdS/CFT) correspondence
pomeranchuk instability
Non-Fermi liquid
quantum phase transition
Thu, 16 Jan 2014 17:59:34 GMTUltracold fermionic atoms in disordered potentials
http://hdl.handle.net/2142/46688
Ultracold fermionic atoms in disordered potentials
Kondov, Stanimir
This thesis describes the construction of an experimental apparatus to study disorder physics using ultracold fermionic atomic gases. We use this apparatus to realize 3D Anderson localization for the first time for quantum matter waves. We provide the first measurement of how the mobility edge—a hallmark of 3-dimensionality—and of localization lengths depend on the disorder strength. In a second experimental study, we add an optical lattice to the disorder potential to realize the disordered Fermi-Hubbard model, which is a minimal model for strongly correlated electronic solids. The interplay of interactions and disorder is investigated. We find an Anderson-like disorder-driven metal-to-insulator transition as well as disorder-induced breaking of the Mott gap in the strongly interacting regime.
ultracold
Fermion
potassium
Anderson
metal-to-insulator transition
Hubbard
Thu, 16 Jan 2014 17:59:07 GMTPairing symmetry, competing orders, and quantum criticality in iron based superconductors via London penetration depth measurements
http://hdl.handle.net/2142/46664
Pairing symmetry, competing orders, and quantum criticality in iron based superconductors via London penetration depth measurements
Salovich, Nicholai
The temperature dependence of the magnetic penetration depth (λ) has been measured in single crystals of BaFe2As2 that have been driven into superconductivity by several different kinds of dopants, specifically potassium, cobalt, and phosphorous.
In (Ba{0.6}K{0.4})Fe2As2 the low temperature behavior of unirradiated samples was consistent with a fully gapped superconducting state with a minimum energy gap Δ{min}/k^B T^C≈ 1. At very high levels of heavy ion irradiation (a column-column separation of 10 nm) a T^2 power law was observed below T^C/3, most likely due to elevated scattering. Neither the location nor the sharpness of the superconducting transition was affected by irradiation. This is evidence for an s{+−} pairing state.
In Ba(Fe{1-x}Co^x)^2As^2 an aluminum coating procedure was employed to extract the zero-temperature value of the in-plane penetration depth λ{ab}(0) as a function of the cobalt concentration x, as it was varied through both the underdoped and overdoped regions of the phase diagram. A pronounced increase in λ{ab}(0) was found as the doping value was decreased below the optimal level. This is evidence for direct competition between the itinerant antiferromagnetic phase and superconductivity that region of the phase diagram.
In BaFe^2(As{1-x}P^x)^2 the same aluminum coating procedure was employed to measure λ{ab}(0) as a function of phosphorous doping. A sharp peak in the penetration depth was found at optimal doping, where the superconducting transition temperature reaches a maximum. This may arise from quantum fluctuations associated with a quantum critical point buried beneath the superconducting dome.
Superconductivity
London Penetration Depth
Iron based superconductors
pairing symmetry
quantum criticality
Thu, 16 Jan 2014 17:58:09 GMTDefects, topology, and the geometric phase in condensed matter physics
http://hdl.handle.net/2142/46645
Defects, topology, and the geometric phase in condensed matter physics
Roy, Abhishek
This thesis presents work on some topological applications in condensed matter physics, particularly geometric phases and defects.
The first chapter deals with pentagonal disclinations in graphene and their
associated bound states. This problem had been attacked previously, using index
theorems, as well as the solutions of the continuum Dirac equation. We
demonstrated \cite{roygraph} that these two approaches as well as bare numerical computation could be made consistent once one took into account boundary conditions at the defect site. For example, the continuum model considers two pentagons and a square to be alike. However there is a physical distinction between the two, as the sublattice symmetry is locally broken in
the former case.
The next chapter treats Berry phases using the Majorana representation of spin
states as points on a sphere \cite{roymajorana}. The advantages of this approach are that one has
a visual representation of the evolution of a state, which automatically absorbs the
gauge freedom that drops out of a geometric phase. I show how non-abelian phases can be treated in this framework.
The third chapter discusses the Kitaev toric code model and its
generalizations, both to higher dimensions and richer braiding symmetries. The toric code
is naturally associated to the mathematical structure
of a chain complex. This leads to a unified treatment of braiding, degeneracy and effective field theory in higher dimensions \cite{roytoric}.
The last part of this thesis is about twist defects in anyonic models. I discuss a general notion of a group defect that permutes anyons and use the
toric code as well as a new honeycomb model \cite{roytwist}, as examples. I discuss fusion of these defects as well as ground state degeneracy. The latter is treated geometrically using covering spaces.
A common theme running through this work is that topological phenomena can be grounded in a lattice model. This makes them more approachable and often clarifies physical details which might otherwise be missed.
condensed matter
topology
geometric phase
defects
anyons
Thu, 16 Jan 2014 17:57:21 GMT